1. Field of the Invention
The present invention relates to an analog-digital (A/D) converter and more particularly to an A/D converter used for an A/D conversion system which receives various analog input signals of different levels and applies these input analog signals to A/D conversion after amplifying them at gains determined depending on the respective levels of the input signals.
2. Description of the Prior Art
In known A/D conversion systems connected through an interface with a data processor and used for converting analog input signals of various different levels into digital signals suitable for processing in the data processor, the input signals are converted into digital signals after amplifying them by an amplifier with a gain which is variable in accordance with the respective ranges of the input signal levels in order to extend the dynamic range of the system for various input signals.
A well-known A/D converter used for such a purpose comprises a variable gain amplifier whose gain is selectively set at either one of 2.sup.0, 2.sup.1, 2.sup.2, . . . , 2.sup.N (N: a positive integer) and a microprocessor for determining the optimum gain corresponding to the level range of each of the input signals, wherein the gain of the amplifier is controlled through a control circuit in accordance with the optimum gain determined by the microprocessor, thus providing an automatic gain control function. Such an A/D converter is disclosed, for example, in the article entitled "What to look for in analog input/output boards" presented by Norman Bernstein in Electronics issued Jan. 19, 1978 or pages 113 to 119. The A/D converter having the automatic gain control function has advantages in that various input signals of different level ranges are subjected to A/D conversion with substantially the same accuracy. In spite of this advantage, since the A/D conversion process for each input signal comprises the steps of subjecting the input signal to A/D conversion by setting the gain of the amplifier at "1" in order to determine the level range of the input signal, determining the optimum gain of the amplifier by the microprocessor in accordance with the level range determined from the digital signal thus obtained, controlling the gain of the amplifier by the control circuit to set it at the optimum gain determined by the microprocessor, and again subjecting the input signal to A/D conversion, it has disadvantages in that it requires a considerably longer time for A/D conversion of each input signal, thus reducing the efficiency of the system.
The recent development of the large scale integrated circuit technique has made it possible to incorporate the A/D converter section and the control circuit in a hybrid IC or a one-chip LSI. Further, the analog input/output system is available on the market as a standardized plug-in unit. In the A/D converter having an automatic gain control as mentioned above, however, the control circuit is quite complicated and therefore, even though the A/D converter section is made up of one chip LSI, the control circuit must be provided in a separate unit. Further, a bus interface must be provided between the microprocessor and the control circuit when arranged on a board, and therefore it leads to the requirements of making complicated interconnections and occupying a large area on the board in the assembly of the A/D converter.
For these reasons, there exists a great demand for an A/D converter wherein the time required for A/D conversion is shortened and the control circuit for gain selection is simplified by simplification of the process for determining the level range of the analog signal, thereby making it possible to incorporate the A/D converter section with the control circuit in a one chip LSI. Further, it is desired that the converter be broadly applicable to various modes of gain selection determined depending on the characteristics of the processes to which the A/D converter is combined by standardizing the essential parts thereof, while making it easy to selectively exchange the remaining parts which are difficult to standarize in accordance with the desired gain select mode.